Long-term, feeder layer-dependent culture systems have been used to identify a number of soluble factors that regulate the final steps in B cell development in murine BM. However, such cultures have provided relatively little information about the regulation of very early B cell development, and even less about prothymocyte development. We have devised a long-term xenogeneic (mouse/rat) lymphoid BM culture system that selectively supports the generation of pro-B cells and prothymocytes from normal and leukemic rat, mouse, and human BM. The most mature cells express the enzyme, terminal deoxynucleotidyl transferase (TdT), which has been implicated in Ig and TCR gene rearrangements. Mouse BM stromal cells and macrophages, as well as a rat BM "accessory" cell population, comprise the essential microenvironment in our culture system. To further characterize this system we will determine: 1) the roles of mouse BM stromal cells and macrophages in the generation of TdT+ BM lymphoid cells; 2) the identity and role of the rat BM "accessory" cell(s) in this process; 3) the microenvironmental requirements for the further differentiation of TdT+ BM cells; and 4) the identities of the responsible growth factors.